DC-DC converters configured as boost or buck or buck/boost converters can use voltage or current sensing modes for determining the magnitude of an output voltage or an output current. In a current sensing mode, the DC-DC converter may sense an output current with a shunt resistor which is coupled in series into an output path. The output current causes a voltage drop across the shunt resistor. The terminals of the shunt resistor are connected to an amplifier the output signal of which can then be used for controlling DC-DC conversion parameters.
FIG. 1 shows a simplified circuit diagram of a conventional DC-DC converter. There is an integrated circuit IC having a control stage CNTL for generating control signals GA1, GA2 for power MOSFETs PM1 and PM2. PM1 is also referred to as high-side (HS) MOSFET and PM2 as low-side (LS) MOSFET. PM1 and PM2 are driven by a HS and a LS driver, respectively. Through power MOSFETs PM1 and PM2, an output inductor L is alternately switched with one side to input voltage (primary supply voltage) and a ground level. An output voltage VOUT is generated which can be higher (boost mode) or lower (buck mode) than the input voltage level VIN. The output voltage VOUT is buffered on a buffer capacitor CBUF. The output voltage level can be sensed through a voltage divider R1, R2 and fed back to the control stage CNTL for adjusting the switching parameters for power MOSFETs PM1, PM2 so as to provide a stable output voltage VOUT of a certain desired voltage level.
A load (not shown) may be coupled to output node VOUT and an output current IL may be supplied to the load. The output current IL causes a voltage drop across the sense (or shunt) resistor RS which is coupled in series to the output inductor L. The voltage difference between pins SA1 and SA2 is sensed with an amplifier AMP. The amplifier AMP amplifies the voltage difference and provides an output signal to control stage CNTL which can then also be used to control the switching parameters of power MOSFETs PM1, PM2. Such a control parameter can be the clock period or the ratio of the ON-period and the OFF-period of a pulse width modulated (PWM) control signal. This can be used for switching the power MOSFETs PM1, PM2. However, conventional amplifiers which are used for current sensing have a limited common mode range, significant offsets which also vary with the common mode input voltage level and low input impedance values which can adversely affect the current sensing by a finite input current into AMP through terminals SA1 and SA2.